The present disclosure relates generally to electrodeless high intensity discharge (HID) lamps. More particularly, this disclosure relates to a ceramic induction HID system and a seal and leg design for hermetically sealing the ceramic induction HID lamp.
An electrodeless or induction high intensity discharge lamp assembly generally includes an arc body located within a central opening of a radio frequency (RF) coil. The coil is typically a multi-turn coil and the arc body is preferably formed of a ceramic whereby the lamp assembly is a substantial improvement over prior quartz arrangements. For example, a ceramic induction HID lamp is believed capable of a lamp life of approximately fifty thousand (50,000) hours. The arc body includes a generally spheroidal portion that in cross-section has the general shape of an ellipse with an elongated, first equatorial axis in one direction and a shorter, second polar axis in a perpendicular direction. At least one ceramic arc body extension or leg extends generally perpendicularly outwardly from the spheroidal portion. Normally the leg is located in a polar region of the spheroidal portion, although there may be other legs located in different regions that communicate with an internal chamber of the spheroidal portion.
The leg is typically hollow and communicates with the internal discharge chamber. It is commonly used as a feedthrough for the ionizing species, and the fill gas of the discharge lamp. In some embodiments of the induction HID lamp, the leg is used for starting purposes and a starting coil is received around the leg. The starting coil is connected to an LC resonant circuit, which provides a start-up or ignition charge to the starting coil. The high voltage coil ionizes the fill and the main RF coil then provides energy to the fill that continues to power the arc discharge, a toroidal-shaped discharge, once ignition of the main fill occurs.
In a traditional HID lamp, an arc tube leg receives an electrically conductive metal electrode. The electrode usually has a crimp or stop surface that locates the electrode in the arc tube leg. Sealing between the metal electrode and the leg is an important consideration and encounters process steps and expensive electrode materials to provide an effective seal design. In addition, the thermal expansion of the metal electrode materials is different from that of the ceramic discharge body, which results in reduced reliability of the seal assembly. The metal electrode is generally a high-temperature refractory metal or cermet material that can handle the electrical feedthrough, high temperatures, and corrosive environment of a metal halide lamp. Commonly, different refractory metals, such as tungsten, molybdenum, and niobium, or cermets of these metals and ceramic materials compatible with the arctube envelope are used. Joining, fabrication, of these materials has many process steps and high cost components.
In the electrodeless or induction HID lamp, there are no electrodes extending into the main discharge chamber. An alternative seal assembly and method to create a robust seal in a ceramic induction lamp are required. A hermetic seal of adequate thickness is required to resist the deleterious impact of the chemical dose or fill. Thus, it is important that the seal member or plug be resistant to chemical attack, and have repeatable construction for ease of assembly. Moreover, the exact location of the seal member within the leg necessarily impacts the volume of the halide dose in the lamp. Again, to enable consistent lamp performance, it is desirable that the dose be closely controlled and likewise this includes provision of a precise, repeatable seal member that is exactly located relative to the leg or arc body.
It would also be preferable if the structure and associated mounting could be simplified. This, in turn, would lead to lower cost in production and materials. Like the remainder of the lamp arc body, it would be preferred if the seal member were made from a non-conductive material, to prevent current flow from inside the arctube to outside, and preferably from the same ceramic used to form the remainder of the arc body. Moreover, a uniform seal thickness between the leg and seal member, and controlled location or insertion length of the seal member within the leg, become important.
Use of a ceramic seal plug is not ideal for all situations. In some instances, a metal is required externally of the body of the discharge lamp to provide electrical connection to the starting coil, for example, and also to mechanically support the electrodeless lamp and mechanically support the starting coil. Thus, an electrode structure with a non-conductive element inside the lamp and conductive element outside the lamp may be preferable.
Accordingly, a need exists for an improved seal and leg design for a ceramic induction lamp.